1. Field of the Invention
The present invention relates to a radioactive gas measurement apparatus for measuring radiation of a radioactive gas and a failed fuel detection system, and in particular, to a radioactive gas measurement apparatus and a failed fuel detection system suitable for measuring Xe-133 emitted when a fuel failure occurs in a reactor.
2. Description of the Prior Art
Detection of a fuel failure in a reactor is accomplished by detecting a radioactive substance in a reactor water or in a gas. In Japanese Patent Laid-Open No. 7-218638, for example, a failed fuel detector that detects the concentration of I-131 in a reactor water is disclosed. This failed fuel detector is configured to measure the concentration of I-131, which is an index for failed fuel detection, by suppressing the effect of a nuclide emitting annihilation gamma rays contained in the reactor water.
FIG. 9 shows an example of a conventional failed fuel detector intended to measure radioactive gas. In this conventional failed fuel detector, a delay tank 41 is provided in a discharge pipe 40 for radioactive gas (primarily containing a bleed air in a reactor condensate system and referred to as an off-gas), and a sampling chamber 43 and an ionization chamber detector 44 both enclosed by a lead shield 42 are provided downstream of the delay tank to monitor the radiation intensity level of the radioactive gas. An index for the failed fuel detection is the concentration of Xe-133 in the radioactive gas. The delay tank 41 is provided because nitrogen-13 contained in a gas in quantity (N-13, having a half-life of 10 minutes and produced in a (p, α) reaction of O-16) interferes with the measurement of the index in the radioactive gas, and without a measure against nitrogen, it is difficult to accurately measure the Xe-133 indicative of the fuel failure. Specifically, this is due to the fact that N-13 emits annihilation gamma rays of 511 keV and the low-energy gamma rays (81 keV) of Xe-133 are hidden in the Compton background thereof. Thus, in order to reduce N-13, a residence time of about 1 hour in the delay tank 41 is provided to remove N-13 before measuring the Xe-133 by the radiation level monitor 44 in the ionization chamber.
Furthermore, in Japanese Patent Laid-Open No. 62-6199 (“Off-Gas Monitor”), there is disclosed a method for determining a quantitative value of Xe-133 by detecting the intensity of gamma rays in the off-gas with a NaI detector and a CaTe detector and processing the value with a computer. In addition, in Japanese Patent Laid-Open No. 3-138593 (“Exhaust Gas Radiation Monitoring Apparatus”), there is disclosed a method in which gamma rays are detected after N-13 is removed from an exhaust gas by taking advantage of the fact that the ion thereof is a negative ion.
In order to detect a fuel failure, an index in a reactor water or a gas needs to be measured quickly and precisely. Therefore, failed fuel detection is desirably conducted by monitoring gas, which exhibits the index earlier than a reactor water. However, in the conventional example shown in FIG. 9, measurement is conducted on the gas after passing through the delay tank, so that the failed fuel detection can only be conducted after about 1 hour. In addition, since a level monitor, such as an ionization chamber, is used for measuring radiation, accurate identification (analysis of nuclide) of Xe-133 is impossible. Besides, the technique described in Japanese Patent Laid-Open No. 62-6199 involves an attenuation pipe for attenuating the radioactivity of the radioactive material having a short half-life before measuring gamma rays of a gas, so that the detection is delayed. According to the technique described in Japanese Patent Laid-Open No. 3-138593, a delay in the detection of gamma rays of Xe-133 due to removal of N-13 is avoided, but the size of the apparatus becomes large.
Alternatively, the failed fuel detector intended for a reactor water described in Japanese Patent Laid-Open No. 7-218638 may be applied to a radioactive gas. In such a case, however, there is a large quantity of annihilation gamma rays of 511 keV from N-13 launched into a main detector (detector for measuring Xe-133) and a Compton scattering component, so that the precision of the analysis of Xe-133 is relatively significantly degraded. This is because the energy of the gamma rays emitted from Xe-133 is 81 keV, which is lower than the energy of the gamma rays emitted from I-131, which is the index for the failed fuel detection in terms of reactor water, of 364 keV. In other words, this is because the lower the energy of an index, the more significant the effect of the Compton scattering component is.